This application is a 371 of PCT/GB94/00045 filed Jun. 11, 1994.
The present invention relates to sensors for the analysis of molten metals and, in particular, to sensors for the measurement of trace elements, such as sulfur, in steel making processes.
Solid electrolyte sensors are based upon the principle that an electric potential difference (voltage) exists across an electrolyte which contains a mobile ion of a given chemical element and which separates two compartments in which the same element has different chemical activities.
The voltage is related to the two chemical activities via the so-called Nernst equation: ##EQU1## where R=the molar gas constant (8.3144 Joule/mole Kelvin);
T=the absolute temperature in Kelvin; PA0 z=the number of electrons transferred in the electromechanical equilibrium under consideration--this is a known value for each system; PA0 F=Faraday's constant (96.485 Coulomb/mole)
and a.sub.melt and a.sub.reference are the two chemical activities.
One of the two chemical activities is fixed by employing a well-known and well-defined chemical system, a so-called reference system, on one side of the electrolyte. Provided the temperature of the two electrolyte interfaces is known (and the same for both interfaces), the voltage across the electrolyte can be directly related to the unknown chemical activity. This chemical activity in turn can be related to the concentration of the element.
Sensors based upon stabilised zirconia solid electrolytes are routinely used in the steel and copper industries to measure the concentration of oxygen in the molten metals. At the present time, the most widely used sensor for molten steel is the so-called "Celox" probe made by Electro-Nite which is a dip-sensor engineered to have a very short lifetime of about one minute in molten steel. The end of the sensor which is dipped into the molten steel comprises a dense cardboard tube with a ceramic and in which the sensor electrolyte is mounted.
GB-A-2196430 describes a continuous sensor for molten steel in which a heat buffer of a refractory material is incorporated between an inner tube of a refractory material in which the measuring element is mounted, and an outer tube which surrounds the inner tube at least over the sensor portion which is to be dipped into the molten metal. The refractory material prevents large temperature variations of the measuring element and thus also prevents large temperature differentials between the portion of the measuring element dipped into the molten metal and the portion of the measuring element above the molten metal.
Various electrochemical sensors for other elements in molten metals are undergoing development and include aluminium in steel and zinc, silicon in steel and pig iron, sulfur in pig iron and copper, phosphorus in pig iron and copper, chromium in steel, sodium in aluminium, copper in copper-tin, calcium in lead-calcium and lithium in aliminium-lithium. The main obstacle in the development of such devices is engineering a device that can withstand both the high temperature involved and the chemically aggressive environments.
For example, GB-A-1470558 uses as the solid electrolyte sodium .beta.-alumina the electrolyte conducting by the movement of sodium ions through the matrix. Although sodium .beta.-alumina is chemically a suitable solid electrolyte for the measurement of sulfur for example in iron or steel, because the sodium ions react with the sulfur ions in the steel to form sodium sulfide according to the equation: ##STR1## in practice, sensors constructed using sodium .beta.-alumina as the solid electrolyte suffer from severe thermal shock when immersed, for example, in molten steel and will thus break in use. The .beta.-alumina of lithium, potassium, rubidium, copper, silver, thallium and gallium are also disclosed for use in this prior art method and apparatus.